Method or transporting a liquid for atomization and a method and devices for atomizing the same

ABSTRACT

Ultrasonic nozzle devices without a central channel but employing a design of cascaded multiple Fourier horns in resonance produce micrometer-sized monodisperse or narrowly-sized droplets with greatly reduced electrical drive power requirements. The liquid to be atomized is brought externally to or adjacent to the endface of the nozzle tip. The above liquid transport method is equally applicable to the ultrasonic nozzle-array devices that are comprised of a plurality of ultrasonic single-nozzle devices configured in parallel. The longitudinal length, transverse width, shape, and area of the nozzle endface of single-nozzle and nozzle-array devices may be tailored or designed (e.g. enlarged) to obtain optimum or large quantities of product droplets to achieve high throughput. By increasing the drive frequency to 8 MHz or higher, sub-micrometer-sized monodisperse or narrowly-sized droplets can be produced using the ultrasonic single-nozzle and nozzle-array devices or any solid endface.

RELATED APPLICATIONS

The present application is related to U.S. Provisional PatentApplication Ser. No. 61/220,964 filed on Jun. 26, 2009, which isincorporated herein by reference and to which priority is claimedpursuant to 35 USC 119.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under grant 5R21EB006366awarded by the National Institute of Health. The government has certainrights in the invention

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of production of micrometer- andsub-micrometer-sized monodisperse or narrowly-sized droplets andtransport of the liquid feed. Monodisperse or narrowly-sized dropletsless than 10 μm in diameter in sprays are highly desirable innanoparticles synthesis from heat sensitive precursor solutions becausethey can be processed at a conveniently low temperature and atmosphericpressure. Also, monodisperse or narrowly-sized droplets (particles) 1 to6 μm in diameter have multiple biomedical applications includingpulmonary drug delivery, micro encapsulation of drugs, and drugpreparation for inhalation. Other potential applications includethin-film coating and three-dimensional (3-D) spray coating for micro-and nano-electronics and photonics.

2. Description of the Prior Art

Droplet (or drop) formation based on acoustic techniques has been animportant research and development subject area in recent years becauseof its various potential applications. For example, nozzle-less dropletejectors that use focused acoustic waves at 5-900 megahertz (MHz) toeject micrometer-sized droplets, one drop at a time, are applicable tohigh-resolution ink-jet printing. A micromachined ultrasonic dropletgenerator based on a liquid horn structure is capable of dropletejection from 5- to 10-μm orifices at multiple resonant frequenciesbetween 1 and 5 MHz and has been shown to be applicable to inhalationdrug therapy. Piezoelectrically actuated flextensional micromachinedultrasonic transducers with annular piezoelectric disk and operatingresonance frequencies from 450 kHz to 4.5 MHz have also been designedand fabricated for droplet ejection. Current ultrasonic nebulizers thatuse vibrating micrometer-sized mesh technology are also capable ofproducing micrometer-sized droplets for applications such as pulmonarydrug delivery, but the resulting drop size distributions are ratherbroad because various atomization mechanisms such as cavitation,impinging, and jetting are also involved in addition to the capillarywave mechanism. Furthermore, the electrical drive power required istypically greater than 10 W. Finally, commercially available metal-basedbulk-type ultrasonic nozzles (Sono-Tek Corp., Milton, N.Y.) that usecapillary waves at frequencies far below 1 MHz are capable of producingsprays of droplets (mist) with a diameter smaller than the nozzleorifice diameter, but much larger than 10 μm. All of the aforementioneddevices under research and development and commercial devices are eitherincapable of producing monodisperse or narrowly-sized droplets of suchdesirable size range (1 to 6 μm) or producing them only at very lowthroughput. Furthermore, they are bulky, requiring much higherelectrical drive power, and do not have potential for mass productionusing batch fabrication. Clearly, it is desirable to explore and realizeminiaturized ultrasonic nozzle devices that operate at megahertz drivefrequencies for producing sprays of micrometer-sized or evensub-micrometer-sized droplets with narrow size distributions, or evenmonodisperse droplets, at high throughput and low electrical drivepower.

Additionally, several attempts have been made in the prior art totransport the liquid to be atomized through the ultrasonic nozzle inwhich it is to be ejected from. Typically, a central channel was used inthe prior art to transport the liquid to be atomized to the endface ofthe nozzle tip of the single-nozzle ultrasonic device. Specifically, inthe Sono-Tek metal-based bulk-type ultrasonic nozzle referred to earliera cylindrical channel is bored through the nozzle body. In thesilicon-based multiple Fourier horns ultrasonic device a pair ofidentical nozzles each with an etched trough along the nozzle axis wasbonded to form a 200 μm×200 μm central channel for transport of theliquid to the endface of the vibrating nozzle tip. As the amplitude ofvibration on the endface of the nozzle tip in the directionperpendicular or nearly perpendicular to the endface reaches a thresholdor critical vibration amplitude at the MHz resonance frequency of themultiple Fourier horns, the liquid resting on the endface is broken intomicrometer-sized monodisperse or narrowly-sized droplets. Formation ofthis central channel requires several additional micro-fabricationsteps. Furthermore, as the operating frequency of the nozzle increases,the physical size of the device decreases. As a result, the area of thevibrating endface decreases, and the degree of complexity involved inchannel fabrication increases. Finally, the corresponding reduction inthe channel cross section will result in decreased liquid flow rate and,thus, droplet throughput.

What is needed is a method and device or devices that eliminate the needfor a central channel and its corresponding fabrication complexitieswhile at the same time maintaining a high flow rate and ahigh-throughput production of monodisperse or narrowly-sized droplets inmicrometer and sub-micrometer sizes, and low electrical drive power.

BRIEF SUMMARY OF THE INVENTION

The miniaturized silicon-based megahertz (MHz) ultrasonic nozzle devicespresented herein solve these problems and more by employing a design ofmultiple Fourier horns in resonance which activates a pure capillarywave atomization mechanism and produce monodisperse or narrowly-sizedmicrometer- and sub-micrometer-sized droplets with greatly reducedelectrical drive power requirements.

Silicon-based ultrasonic nozzles have several advantages overconventional metal-based bulk-type ultrasonic nozzles found in the priorart. Silicon possesses a relatively large electromechanical couplingcoefficient. More importantly, mass production of any similar ordifferent nozzle profiles can be readily accomplished using thewell-established inductive coupled plasma (ICP) process ofmicroelectromechanical system (MEMS)-based fabrication technology.

We have recently established that the underlying physical mechanism foratomization or droplets generation, namely, temporal instability ofFaraday waves (also called standing capillary waves) is related to the“critical vibration amplitude” in the direction perpendicular or nearlyperpendicular to the endface of the nozzle tip to form a layer of liquidon the endface, but is not related to the means of liquid transport tothe endface of the nozzle tip or the device configuration or method usedfor excitation of the critical vibration amplitude. As a result of theabove basic concept, the central channel of the prior art may bereplaced by simple means of externally bringing the liquid to theendface of the nozzle tip. For example, tubing such as fused silica,Teflon®, metal, or a light wicker connected to the source of liquid atone end and in touch with or close to the endface of the nozzle tip atits opposing end will serve the purpose. Thus, it must be understoodthat “tubing” within this specification and its claims shall mean anymeans, mechanism, micropiping, channel, conduit or device fortransporting liquid, nanoparticles dispersion, or other material to beatomized from a source of the same to or near the nozzle endface. Theabove basic concept further suggests that device configurations such asa single nozzle alone without a central channel and a simple solidendface vibrating with corresponding “critical amplitude” at a givendrive frequency may be used to produce monodisperse or narrowly-sizeddroplets. It should also be noted that the resonance effect among themultiple Fourier horns of a single-nozzle device readily generates therequired “critical vibration amplitude” for atomization of the liquidresting on the endface of the nozzle tip at a low electrical drivepower.

Accordingly, elimination of the central channel greatly simplifies theMEMS-based micro fabrication steps for the single-nozzle devices andthus, reduces their ultimate manufacturing costs. The above liquidtransport method is equally applicable to the ultrasonic nozzle-arraydevices that are comprised of a plurality of ultrasonic single-nozzledevices configured in parallel.

The longitudinal length, transverse width, shape, and area of the nozzleendface of single-nozzle and nozzle-array devices may be tailored ordesigned (e.g. enlarged) to obtain optimum or large quantities ofproduct droplets to achieve high throughput. Replacement of the centralchannel with judicious design of the endface or end plate of the nozzletip in length, width, shape, and area facilitates direct transport of alarge quantity of the liquid to the endface and, thus, achieveshigh-throughput of product droplets for many applications such asinhalation or pulmonary drug delivery, micro encapsulation of drugs,thin-film coating, three-dimensional (3-D) spray coating for micro- andnano-electronics and -photonics, and nanoparticles synthesis. It is tobe expressly understood that the liquid referred to in the currentinvention includes pure-substance liquid, solution, and nanoparticlesdispersion.

The method of the illustrated embodiments are applicable to anyvibrating solid surface provided that its vibration amplitude(displacement) in the direction perpendicular or nearly perpendicular tothe surface reaches the corresponding “critical amplitude” or higher fora given drive frequency of vibration. Since the droplet diameter isinversely proportional to the drive frequency to the 2/3 power, byincreasing the drive frequency to 8 MHz or higher, sub-micrometer-sizedmonodisperse or narrowly-sized droplets may be produced.

Such an advantage is applicable to the ultrasonic devices which utilizean array of single-nozzle device configured in parallel. The method forliquid transport facilitates high-throughput production of micrometer-and sub-micrometer-sized monodisperse or narrowly sized droplets forinhalation or pulmonary drug delivery. Current devices (e.g.,nebulizers, metered dose and dry powder inhalers) all suffer from broaddroplet or particle size distributions and low throughput, which make itdifficult to deliver sufficient drug to targeted sites precisely andrapidly. In intubated patients, polydisperse aerosols also limitdelivery through the ventilator tubing for adults and especially inneonates. Thus, there is a need for a droplet (aerosol) device thatproduces more uniform or even monodisperse droplets or aerosols withincreased throughput of drug delivery to reduce treatment time, smallphysical size for easy access to target, and low electrical drive power.The illustrated embodiments of the invention facilitate realization of aminiaturized ultrasonic high-throughput micron- and submicron-sizedmonodisperse droplet device to fulfill such an unmet need. Therefore inone particular embodiment, the device of the current invention may bebattery powered and pocket sized in order to function as a miniaturizedmonodisperse medicinal nebulizer for popular and outpatient use.

While the device and method has been or will be described for the sakeof grammatical fluidity with functional explanations, it is to beexpressly understood that the claims, unless expressly formulated under35 USC 112, are not to be construed as necessarily limited in any way bythe construction of “means” or “steps” limitations, but are to beaccorded the full scope of the meaning and equivalents of the definitionprovided by the claims under the judicial doctrine of equivalents, andin the case where the claims are expressly formulated under 35 USC 112are to be accorded full statutory equivalents under 35 USC 112. Theinvention can be better visualized by turning now to the followingdrawings wherein like elements are referenced by like numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a silicon single-nozzle deviceconsisting of a drive section and a resonator section with three Fourierhorns, the most distal Fourier horn comprising a normal nozzle tip orendface.

FIG. 2 is a perspective view of two silicon single-nozzle devices seenin FIG. 1 combined together in a nozzle-array device, wherein each ofthe single-nozzle devices comprises an enlarged or “hammer head” nozzletip or endface.

FIG. 3 is a schematic diagram of an atomization apparatus employing thesilicon single-nozzle device seen in FIG. 1.

FIG. 4 is a graph of size distribution in terms of probability densityand cumulative undersize percentage versus droplet diameter for water(plot (a) at 1.0 MHz) and alcohol droplets (plot (b) at 2.0 MHz and plot(c) at 2.5 MHz) produced by the silicon single-nozzle device seen inFIG. 1 and the size distributions of droplets produced by two commercialnebulizers (plots (d) and (e)).

FIG. 5 is a magnified perspective view of the enlarged or hammer headnozzle tip or endface seen within the nozzle-array device of FIG. 2.

FIG. 6 is a perspective view of a silicon-nozzle device with fourFourier horns, the most distal Fourier horn comprising an enlarged orhammer head nozzle tip or endface.

FIG. 7 is a schematic diagram of the platform for a miniaturizedultrasonic droplet generator employing the ultrasonic nozzle-arraydevice seen in FIG. 2.

FIG. 8 is a perspective view of an alternative embodiment of thesingle-nozzle device seen in FIG. 1 with the most distal Fourier hornhaving longitudinal displacement magnification of one and, thus, arectangular shaped end piece with an entrenched area for liquid feedingand large endface.

The invention and its various embodiments can now be better understoodby turning to the following detailed description of the preferredembodiments which are presented as illustrated examples of the inventiondefined in the claims. It is expressly understood that the invention asdefined by the claims may be broader than the illustrated embodimentsdescribed below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The miniaturized ultrasonic nozzle device 1 of the current invention iscomprised of a silicon single-nozzle device, generally denoted withreference numeral 10 as seen in FIG. 1, supported by two silicon strips,one on each side (not shown) and coupled to the nodal bars 24 that aredisposed laterally through the silicon single-nozzle device 10. It is tobe expressly understood that the single-nozzle device 10 in FIG. 1 isequivalent to the miniaturized ultrasonic nozzle device 1, and is usedhenceforth for brevity. The single-nozzle device 10 comprises a drivesection 12 and a resonator section 14 in a common silicon substrate 16that is made of one or more pieces of silicon wafers. The drive section12 comprises a piezoelectric transducer such as lead zirconate titanate(PZT) 18 coupled to the rectangular shaped base of the silicon substrate16 using a silver paste as is known in the art. It is to be expresslyunderstood, however, that other forms of bonding such as welds, alloys,or other pastes or resins now known or later devised may also be usedwithout departing from the original spirit and scope of the invention.

In one embodiment, the resonator section 14 of each siliconsingle-nozzle device 10 comprises three Fourier horns 20. Each Fourierhorn 20 is half a wavelength long with a longitudinal vibrationamplitude (displacement) magnification of two. Other magnificationssmaller than two may also be used without departing from the spirit andscope of the invention. The drive section 12 and each Fourier horn 20also comprise a nodal bar 24 that is disposed laterally through thesilicon single-nozzle device 10. The most distal Fourier horn 20 in theresonator section 14 comprises a normal nozzle tip or endface 22.

Excitation of the PZT transducer plates 18 by an AC voltage at thenozzle resonant frequency creates a standing acoustic wave along thesingle-nozzle device 10 with a maximum longitudinal vibration(displacement) at the nozzle tip or endface 22 of the siliconsingle-nozzle device 10. The resonance effect of the multiple Fourierhorns 20 greatly enhances the longitudinal displacement on the nozzleendface 22. As a result of the vibration, Faraday waves are formed onthe free surface of the liquid layer resting on the nozzle tip orendface 22. Subsequent breakup of the Faraday waves results inatomization and production of monodisperse or narrowly-sized droplets.

The silicon ultrasonic single-nozzle device 10 is preferably fabricatedusing MEMS technology. The ultrasonic single-nozzle device 10 isfabricated according to the desired resonant frequency to be used, thedimensions of the single-nozzle device 10 being larger for when arelatively low resonant frequency is to be used, and smaller dimensionsfor when a higher resonant frequency is to be used. For example, inorder to have a relatively low resonant frequency of 0.5 MHz thedimensions of the single-nozzle device 10 may be 3.66 cm×0.38 cm×0.11 cmwhile for a higher resonant frequency of 1.5 MHz, the dimensions of thesingle-nozzle device 10 may be 1.20 cm×0.15 cm×0.05 cm. These dimensionsof the single-nozzle device 10 are meant to be illustrative purposesonly. Other substantially similar dimensions for the single-nozzledevice 10 may also be used in order to obtain substantially similarresonant frequencies without departing from the original spirit andscope of the invention.

Liquid from an outside source (not seen) flows through a tubing 28 andissues onto the nozzle tip or endface 22 which vibrates longitudinallyat the nozzle resonance frequency. The tubing 28 may be comprised ofmetal or metal alloys, plastic or plastic composites, or a light wickerwith its distal end in contact with or in close proximity to the endfaceof the nozzle tip 22. When the vibration amplitude of the nozzle endface22 exceeds a threshold (critical amplitude), a liquid layer ismaintained on the nozzle endface 22 and the Faraday waves, formed on thefree surface of the liquid layer, grow exponentially in amplitude,resulting in generation of droplets.

Atomization of the liquid is carried out at room temperature (20° C.)using, for example, deionized (DI) water, 0.25% nonionic surfactant(Triton X-100) solution, ethanol (ethyl alcohol), aqueous glycerolsolution, aqueous and ethanol solutions of isoproterenol (salbutamol oralbuterol), insulin, and aqueous gold nanoparticles dispersion. Becausethe single-nozzle device 10 takes advantage of the resonance effect, theelectric drive power required for atomization is too low to cause anysignificant increase in temperature. In fact, the temperature in thedrive section 12 at an electrode voltage even higher than that requiredfor atomization was measured to increase by less than 2° C., and nochange in the temperature of the atomization liquids, was detected by athermocouple.

When the ultrasonic single-nozzle device 10 shown in FIG. 1 is driven atits resonant frequency of 971.1 kHz by the drive system 32 and PZTtransducer 18, each of the three Fourier horns 20 in cascade in eachultrasonic single-nozzle device 10 vibrates longitudinally with neitherflexural nor lateral motion. Furthermore, the maximum magnitude oflongitudinal vibration or displacement at each succeeding Fourier horn20 tip increases progressively by a factor of 2, resulting in an overallvibration displacement magnification of 8 (theoretical design 2³=8) onthe nozzle tip or endface 22.

The sizes and size distributions of the droplets produced by theultrasonic single-nozzle device 10 are measured using laser lightdiffraction technique provided by the Malvern/Spraytec System (Model#STP5311). The measured results for the water droplets produced by a 1.0MHz and for the alcohol droplets produced by a 2.0 MHz and a 2.5 MHzsingle-nozzle device 10 are shown in FIG. 4. The droplets are clearlymonodisperse with geometrical standard deviation (GSD) as small as 1.14and respective MMD of 4.4 μm, 2.4 μm, and 1.6 μm. Note that GSD of 1.0corresponds to single size and GSD up to 1.22 is commonly accepted asmonodisperse in aerosol medicine. MMD is a measure of droplet size andstands for mass median diameter. In addition, the measured electricaldrive power was found as low as 55-160 mW at the droplet throughput of0.42 to 0.65 ml/min. The low drive power requirement made possible bythe concept of multiple Fourier horns 20 in resonance is supported bythe theoretical calculation described in further detail below.

The atomization and production of micrometer-sized monodisperse ornarrowly-sized droplets by the ultrasonic single-nozzle device 10 ismade possible by the temporal instability of Faraday waves, also calledstanding capillary waves. Faraday waves are generated on the freesurface of the liquid resting on the planar or nearly planar endface ofthe nozzle tip when the peak longitudinal excitation displacement (h) ofthe endface (at the drive frequency f) reaches a critical value h_(cr).The amplitude ξ(x,t) of the resulting Faraday waves with initialamplitude ξ_(o) is given by the following equation:ξ(x,t)=ξ_(o) e ^(πkf(h-h) ^(cr) ^()t) sin(2π(f/2)t−π/4)cos kx  (1)In which t designates time and k=2π/λ is the wave number of the Faradaywaves, and the X-axis is perpendicular to the nozzle axis and parallelto the width of nozzle endface. The frequency of the Faraday waves isseen as equal to one half of the drive frequency, and the correspondingFaraday wavelength λ is determined by the Kelvin equation:λ=(8πσ/ρ)^(1/3) f ^(−2/3)  (2)in which f, σ, and ρ are the ultrasonic drive frequency, the surfacetension, and the density of the liquid, respectively. Clearly, theFaraday waves generated become temporally unstable when the peakexcitation displacement h exceeds the critical value h_(cr) for Faradaywave formation given as follows:h _(cr)=2v{ρ/(πfσ)}^(1/3)  (3)where the liquid kinematic viscosity v=μ/ρ in which μ is the liquidviscosity. The amplitude of the Faraday waves at MHz drive frequencygrows rapidly once the excitation displacement h exceeds the criticalvalue h_(cr), and the Faraday waves become unstable, resulting inatomization and production of monodisperse or narrowly-sized droplets.

Finally, the diameter (D_(p)) of the droplets produced is proportionalto the Faraday wavelength λ as given in Equation 4:D _(p) =Cλ  (4)where the proportionality constant C ranges from 0.34 to 0.40.

Close agreement between the predicted and the measured diameters of thedroplets produced by the ultrasonic nozzle devices with a single-nozzledevice 10 operating at 0.5, 1.0, 1.5, and 2.0 MHz is shown in Table 1below. The narrow bandwidth of the atomization frequency made possibleby the novel design of multiple Fourier horns 20 in resonance results inproduction of monodisperse droplets with GSD as low as 1.1.

TABLE 1 Measured droplet diameters are in agreement with predictedvalues of D_(p) = 0.40λ. Drive freq. f Surface Tension Density FaradayDroplet dia. D_(p) (μm) (MHz) σ ρ Wavelength Pred'd (Nominal) Liquid(dyn/cm) (g/cm³) λ^(a)) (μm) 0.40λ Meas'd^(b)) 0.5 Water 72 1.00 19.67.83 7.11 1.0 Water 72 1.00 12.2 4.88 4.64 1.0 Glyc. aq^(c)) 72 1.1111.7 4.71 4.51 1.0 Alcohol 23 0.79 9.0 3.60 3.47 1.5 Water 72 1.00 9.33.72 3.66 1.5 Glyc. aq^(c)) 72 1.10 9.0 3.59 3.73 1.5 Alcohol 23 0.796.9 2.76 2.49 2.0 Water 72 1.00 7.7 3.08 2.89 2.0 Alcohol 23 0.79 5.72.27 2.24 ^(a))Faraday wavelength λ = (8πσ/ρ)^(1/3) f^(−2/3),^(b))Experimental errors: ±0.04 for 0.5, 1.0 and 1.5 MHz; ±0.08 for 2.0MHz nozzles, ^(c))Aqueous solution of 40-44 wt % glycerol with viscosityup to 4.5 cP.

The temporal instability of Faraday waves also supports the very lowelectrical drive power the single-nozzle devices require foratomization. The critical vibration displacement h_(cr) of 0.33, 0.29and 0.26 μm predicted by Equation 3 for water at 1.0, 1.5, and 2.0 MHzdrive frequencies, respectively, are to be compared to the measured peakexcitation displacements of 0.34, 0.32, and 0.31 μm required foratomization using a laser Doppler vibrometer (Polytech GmbH, Model PSV400). The fact that the measured peak excitation displacements are onlyslightly higher than the predicted h_(cr) values verifies that the MHzFaraday waves rapidly become unstable and result in atomization once theexcitation displacement exceeds the critical vibration displacement.Thus, the low peak excitation displacements required for atomizationsupport the low electrical drive power measured for atomization, namely,55 to 160 mW at throughput of 0.35 to 0.65 ml/min with the 2.0, 1.5, and1.0 MHz nozzles. Note that this range of drive power is at least twoorders of magnitude lower than that required in conventional ultrasonicatomization using MHz disk transducers. This very low drive powerrequirement is attributable to the resonance effect of the three Fourierhorns 20 used in the nozzles 10, since only a minute amount of power isneeded to excite a nearly loss-free resonant system. The major sourcesof power losses that must be furnished by the electrical generator arethe vibration of the nozzle endface, the lossy PZT transducer 18, thebonding between the PZT transducer 18 and the silicon substrate 16 inthe drive section 12, and that between the pair of basic nozzles 10required in the formation of a central channel in the case of earliernozzles. The single-nozzle 10 and nozzle-array devices 50 without acentral channel also have the advantage of lower electrical drive powerfor atomization.

In another embodiment, shown in FIG. 6, the single-nozzle device 10comprises four Fourier horns 20 within the resonator section 14. Themost distal Fourier horn 20 is designed with a longitudinal displacementmagnification M ranging from 1.0 to 1.9, and comprises an enlargednozzle tip or endface 26 in width that is larger than the normal nozzletip 22. The addition of a fourth Fourier horn 20 increases the maximummagnitude of longitudinal displacement of the enlarged nozzle endface 26by an additional magnification factor M, resulting in an overallvibration amplitude or displacement magnification or gain of 8M. Thefour Fourier horns 20 with enlarged nozzle endface 26 combine tosignificantly raise the throughput of the monodisperse or narrowly-sizeddroplets which may be produced by the single-nozzle device 10. It shouldalso be understood that fewer or additional Fourier horns 20 other thanwhat is shown in FIG. 6 may also be used without departing from theoriginal scope of the invention.

In an alternative embodiment shown in FIG. 2, two identical siliconsingle-nozzle devices 10 are coupled in parallel to form an ultrasonicnozzle-array device 50. The drive sections 12 and resonator sections 14of each single-nozzle device 10 are aligned and coupled side by sidewith the nodal bars 24 of each single-nozzle device 10 touching thenodal bars 24 of the adjacent single-nozzle device 10 as seen in FIG. 2.The distal Fourier horn 20 of each of the single-nozzle devices 10 ofthe current embodiment, however, comprises an enlarged or “hammer head”nozzle tip or endface 26 as seen in the magnified view of FIG. 5. Theenlarged or hammer head nozzle endface or tip 26 comprises a wider widthcompared to that of the normal nozzle endface 22 and, therefore, mayaccommodate a higher droplet throughput than that of the nozzle tip 22.

It should be stressed that based on the theory of temporal instabilityof Faraday waves summarized above, formation of a liquid layer on thevibrating endface is essential for stable atomization to take place.However, formation of liquid layer is independent of how liquid istransported to the vibrating nozzle endface 22, 26. Therefore, othermeans of transporting a liquid other than by tubing 28 may also be usedwithout departing from the original spirit and scope of the invention.

One such means is an end plate 52 which is coupled to the distal end ofthe Fourier horn 20 to form the hammer head nozzle tip or endface 26 ofeach individual silicon single-nozzle device 10 in the twin nozzle-arraydevice 50 depicted in FIG. 2. The end plate 52 vibrates in unison witheach corresponding single-nozzle device 10 in the directionperpendicular or nearly perpendicular to the surface of the end plate 52along the nozzle axis as seen in the broken line outline in FIG. 2.Since the liquid to be atomized can be transported directly to thesurface of the end plate 52, no central channel for liquid flow isneeded. As liquid is fed onto the surface of the end plate 52, a liquidlayer will be formed on it and stable atomization will take place whenthe vibration amplitude on the surface of the end plate exceeds thecritical value given by Equation 3 above. Since the area of thevibrating hammer head or enlarged nozzle tip 26 comprising end plate 52in contact with the liquid to be atomized is significantly larger thanthe usual tip area of just a nozzle tip alone 22, the nozzle-arraydevice 50 will provide a much higher throughput of monodisperse ornarrowly-sized droplets. The excellent agreement between theexperimental results and the theoretically predicted values based on thetheory of temporal instability of Faraday waves summarized aboveprovides the solid scientific basis for such a new and novelnozzle-array configuration and the resulting devices. In accordance withEquation 4 above, the nozzle-array device 50 will produce 1.0 and 0.9 μmwater droplets and alcohol droplets, respectively, at the operatingfrequency of 8 MHz. In one embodiment, each individual enlarged endplate 52 comprises the capability for simultaneous or sequentialatomization of different liquids and their subsequent mixing desirablefor some applications such as drug delivery to a patient. It should alsobe expressly understood that the number of single-nozzle device 10 inthe nozzle-array device 50 can be readily increased via batchfabrication as commonly known in the art without departing from theoriginal spirit and scope of the invention.

The implementation of the nozzle-array device 50 into a miniaturizedultrasonic droplet generator 60 can be seen in FIG. 7. The miniaturizedultrasonic droplet generator 60 comprises a driver module 62 coupled tothe PZT transducers 18 of the nozzle-array device 50, wherein the drivermodule 62 comprises a function generator and amplifier on a smallprinted circuit board. The driver module 62 also comprises means foraccurate setting and tuning of the drive frequency, control of theoutput power, and continuous or burst-mode of operation of theminiaturized droplet generator 60. The driver module 62 is small enoughso that it may function as a convenient plug-in or battery powered unitas is known in the art.

The miniaturized ultrasonic droplet generator 60 also comprises aplatform 56 which supports the nozzle-array device 50 and a plurality oftubing 28 which delivers the liquid to be atomized to the end plates 52of the nozzle-array device 50. The platform 56 is coupled to the drivermodule 62 at its proximal end and to a mouth piece 54 at its distal endas depicted in FIG. 7. The mouth piece 54 has the general shape of afunnel and is used to direct the atomized liquid produced by thenozzle-array device 50 into the mouth and in turn the respirationsystem, for example, of a patient. Because of the low power demands ofthe nozzle-array device 50, the entirety of the miniaturized ultrasonicdroplet generator 60 is small enough to be hand held or evenpocket-sized and is powered by conventional batteries as is known in theart,

In addition to each hammer head or enlarged nozzle tip 26 comprising itsown end plate 52, in a separate embodiment, a single or common end plate(not shown) may also be coupled to some or all single-nozzle devices 10contemporaneously within the nozzle-array device 50 and thus furtherincrease the throughput of the monodisperse or narrowly-sized droplets.

In still a further embodiment, the normal nozzle tip 22 of thesingle-nozzle device10 seen in FIG. 1 may be enhanced by coupling anenlarged end piece 64 to the distal Fourier horn 20 as seen in FIG. 8.The end piece 64 comprising a trenched area 67 proximal to its largerectangular shaped nozzle endface 66 and vibrating in resonance withFourier horns 20 is coupled to the distal Fourier horn 20 by means knownin the art and comprises a nodal bar 24 disposed across its width sothat it may be paired with at least one other single-nozzle device 10 ina nozzle-array device 50 similar to what is seen in FIG. 2. Liquid to beatomized is delivered to the trenched area 67 of the end piece 64 viathe tubing 28. The endface 66 provides a much larger area foratomization to take place as compared to that of the normal nozzle tip22 and enables the single-nozzle device 10 to dramatically increase thethroughput of droplet production.

Replacement of the central channel for liquid flow found in the priorart in each individual single-nozzle device 10 by external liquid feedvia a tubing 28 to the normal nozzle endface 22 or the enlarged nozzleendface 26, 66 eliminates the additional fabrication steps required inconstructing the central channel and will, in turn, significantly lowerthe ultimate manufacturing costs of single-nozzle devices 10 andnozzle-array devices 50.

Thus, the single-nozzle device 10 and the nozzle-array device 50 arecapable of providing all the desirable features enumerated at the outsetabove, namely, monodisperse or narrowly-sized droplets with optimum sizerange (1 to 6 μm), high throughput and thus reduced treatment time,small physical size for easy access to target, and very low electricaldrive power. These desirable features, together with the aforementionedunique capabilities, should facilitate development of new technologiesfor systemic therapy via the lung by absorption through alveoli.

Many alterations and modifications may be made by those having ordinaryskill in the art without departing from the spirit and scope of theinvention. Therefore, it must be understood that the illustratedembodiment has been set forth only for the purposes of example and thatit should not be taken as limiting the invention as defined by thefollowing invention and its various embodiments.

Therefore, it must be understood that the illustrated embodiment hasbeen set forth only for the purposes of example and that it should notbe taken as limiting the invention as defined by the following claims.For example, notwithstanding the fact that the elements of a claim areset forth below in a certain combination, it must be expresslyunderstood that the invention includes other combinations of fewer, moreor different elements, which are disclosed in above even when notinitially claimed in such combinations. A teaching that two elements arecombined in a claimed combination is further to be understood as alsoallowing for a claimed combination in which the two elements are notcombined with each other, but may be used alone or combined in othercombinations. The excision of any disclosed element of the invention isexplicitly contemplated as within the scope of the invention.

The words used in this specification to describe the invention and itsvarious embodiments are to be understood not only in the sense of theircommonly defined meanings, but to include by special definition in thisspecification structure, material or acts beyond the scope of thecommonly defined meanings. Thus if an element can be understood in thecontext of this specification as including more than one meaning, thenits use in a claim must be understood as being generic to all possiblemeanings supported by the specification and by the word itself.

The definitions of the words or elements of the following claims are,therefore, defined in this specification to include not only thecombination of elements which are literally set forth, but allequivalent structure, material or acts for performing substantially thesame function in substantially the same way to obtain substantially thesame result. In this sense it is therefore contemplated that anequivalent substitution of two or more elements may be made for any oneof the elements in the claims below or that a single element may besubstituted for two or more elements in a claim. Although elements maybe described above as acting in certain combinations and even initiallyclaimed as such, it is to be expressly understood that one or moreelements from a claimed combination can in some cases be excised fromthe combination and that the claimed combination may be directed to asubcombination or variation of a subcombination.

Insubstantial changes from the claimed subject matter as viewed by aperson with ordinary skill in the art, now known or later devised, areexpressly contemplated as being equivalently within the scope of theclaims. Therefore, obvious substitutions now or later known to one withordinary skill in the art are defined to be within the scope of thedefined elements.

The claims are thus to be understood to include what is specificallyillustrated and described above, what is conceptionally equivalent, whatcan be obviously substituted and also what essentially incorporates theessential idea of the invention.

We claim:
 1. An ultrasonic single-nozzle device for atomizing a liquidinto monodisperse or narrowly-sized droplets comprising: a resonatorsection including a plurality of Fourier horns disposed in a cascadedconfiguration, wherein each of the Fourier horns comprises a nodal bardisposed across its width; a nozzle endface disposed on the distal endof the most distal Fourier horn within the cascaded configuration;tubing for delivering the liquid to be atomized disposed adjacent to orin contact with the nozzle endface; and a drive section comprising apiezoelectric transducer such as lead zirconate titanate (PZT)transducer coupled to the resonator section and a nodal bar disposedacross its width.
 2. The ultrasonic single-nozzle device of claim 1where the resonator section comprises three Fourier horns disposed in acascaded configuration.
 3. The ultrasonic single-nozzle device of claim2 further comprising a rectangular end piece with a trenched area andenlarged endface coupled to the most distal Fourier horn within thecascaded configuration.
 4. The ultrasonic single-nozzle device of claim1 where the resonator section comprises four Fourier horns disposed in acascaded configuration.
 5. The ultrasonic single-nozzle device of claim4 further comprising an end plate coupled to the nozzle tip toeffectively provide an increased surface area of the nozzle tip ascompared to the nozzle tip without the end plate.
 6. The ultrasonicsingle-nozzle device of claim 4 further comprising a rectangular endpiece with a trenched area and enlarged endface coupled to the mostdistal Fourier horn within the cascaded configuration.
 7. The ultrasonicsingle-nozzle device of claim 1 further comprising a function generatorand an amplifier coupled to the piezoelectric transducer, the functiongenerator and amplifier comprising means for driving the nozzle at itsresonance frequency.
 8. An ultrasonic nozzle-array device for atomizingat least one liquid into monodisperse or narrowly-sized dropletscomprising: at least two single-nozzle devices configured in parallel;each of the single-nozzle devices including a plurality of Fourier hornsdisposed in a cascaded configuration, wherein each of the Fourier hornscomprise a nodal bar disposed across its width; a nozzle endfacedisposed on the distal end of the most distal Fourier horn within thecascaded configuration of each of the at least two single-nozzledevices; tubing for delivering the liquid to be atomized disposedadjacent to or in contact with each of the nozzle endfaces of each ofthe single-nozzle devices; and a piezoelectric transducer coupled to thebase of a drive section that is coupled to the first Fourier horn ofeach of the at least two single-nozzle devices.
 9. The ultrasonicnozzle-array device of claim 8 where the at least two single-nozzledevices configured in parallel comprises the at least two single-nozzledevices adjacently coupled so that the nodal bars of each of theplurality of Fourier horns of each of the at least two single-nozzledevices are in contact or mechanically coupled.
 10. The ultrasonicnozzle-array device of claim 8 further comprising a function generatorand an amplifier coupled to the piezoelectric transducer on the at leasttwo single-nozzle devices, the function generator and amplifiercomprising means for driving the at least two single-nozzle devices attheir resonance frequencies.
 11. The ultrasonic nozzle-array device ofclaim 8 further comprising an end plate coupled to the nozzle tip ofeach of the at least two single-nozzle devices to effectively provide anincreased surface area of the nozzle endface of the at least twosingle-nozzle devices as compared to the nozzle endface without the endplate.
 12. The ultrasonic nozzle-array device of claim 8 furthercomprising a platform disposed around the nozzle-array to support thetubing and a mouth piece to direct the atomized liquid from thenozzle-array device to the mouth of a user.